This is a recurring argument of atheists/agnostics. You don't have to prove God to be true, God is already proved by your sheer existence, your breathing, your being alive. What keeps you alive? Your heart pumping fresh blood to all parts of your body. What keeps your heart beating? Each day the average heart "beats" (or expands and contracts) 100,000 times and pumps about 2,000 gallons of blood. In a 70-year lifetime, an average human heart beats more than 2.5 billion times. It never stops beating until it fails and the result is usually death if it is not restarted quickly. The source of energy for the heart, like all other tissues is ATP or adenosine triphosphate. But keeps this ATP in existence? The food you digest. What makes you digesting food? It is definitely not YOU, since you can't control the process of digestion, you were never taught "how to digest food properly", and still your body does it without further commands. You need no further evidence, but you can go on like this, ask a thousand more question, and will finally come to the same question: what is the Source of Being? You cannot answer that question on an atheist / agnostic prejudice.

On the other hand, God is not proved by theoretical arguments, it is proved by human life experience. Jesus wasn't preaching about the "existence of God", he told us that he brought good news, he healed the sick people, sick in mind or body. He brought love where there was hatred, he brought certainty where there was shakiness. He made miracles, where miracles were needed. With a simple but profound touch, Jesus broke down barriers, challenges customs and laws that alienate, and embodied his convictions about the inclusive meaning of the Reign of God. Moreover Jesus didn't care about proving the existence of God to men, since it is blatantly obvious, he meant to bring the power of God to the people.